This invention relates to devices of the type which control the flow of conditioned air through a duct into a controlled space in response to a sensed condition such as temperature, humidity, and the like.
In such devices an air damper in the duct is operated by an actuator in response to the sensed condition. In more sophisticated proportioning installations, the volume flow of air is controlled in response to the sensed condition. The volume flow of air is maintained constant at any one sensed demand for conditioned air and changes only in response to a change in sensed demand. The position of the damper in response to a sensed demand for conditioned air is varied in response to a sensed flow of conditioned air through the duct to maintain the volume flow constant. Since flow is a function of pressure difference, it may be measured as a function of the difference in pressure between two different locations along the stream of air, preferably above and below a restriction.
When the sensed condition or the condition set point changes, the sensed demand changes, resulting in the actuator repositioning the damper to permit fulfillment of the new demand by increasing or decreasing the flow of conditioned air. This change in air flow through the duct results in a sensed difference in the flow, which difference acts on the damper through the actuator to maintain the flow constant. The change in sensed demand for conditioned air and the sensed change in flow thus counteract each other, producing an oscillatory movement of the damper, permitting flow of alternately more and less air as first one and then the other of the sensed values predominates. The oscillation of the damper not only produces undesirable fluctuation in air flow into the controlled space, but also produces corresponding fluctuating changes in upstream air pressure. Changes in upstream air pressure affect flow through other ducts supplied from the same source of conditioned air, so that dampers in these other ducts may also begin to oscillate. This produces undesired fluctuating air flows into the spaces supplied by these ducts. Furthermore, the resulting changes in damper positions produce a cumulative affect on upstream pressure to further accentuate the oscillations. The undesired affects therefore may be both progressive and cumulative. Until now, no solution to this serious problem has been available.